"The courage and hope embodied by Malala and girls like her around the world should serve as a call to action," Mrs Obama said.

It is unusual for a US first lady to make outspoken foreign policy remarks, but Mrs Obama has campaigned for the girls' release.

Michelle Obama has often appeared alongside her husband during the weekly address, but this is the first time she has delivered the speech alone.

Earlier this week, she tweeted a picture of herself in the White House holding a sign with the message "#BringBackOurGirls".

The UN Security Council expressed outrage over the abductions, saying it would consider "appropriate measures" against Boko Haram. The US is seeking to have UN sanctions imposed on the group.

Michelle Obama has been actively campaigning for the release of the girls

Western help

Right now, more than 65 million girls worldwide are not in school, yet we know that girls who are educated make higher wages, lead healthier lives, and have healthier familiesMichelle Obama , US First Lady

A senior US official said Washington was also considering a Nigerian request for surveillance aircraft.

British High Commissioner Andrew Pocock said drones could help gather intelligence but urged caution.

He told the BBC's Today programme: "The eye in the sky, even if it were able to be focused on the spot, isn't a panacea."

Traditional hunters armed with bows and arrows and old-fashioned shotguns are ready to enter the forest where the girls are thought to be held, local officials in Borno state have told the BBC's Mark Doyle.

They say 400 to 500 men have gathered but their departure is not imminent - they still hope the army will step up its efforts.

Our correspondent says it is a sign of Nigerians' frustration with the lack of progress in the search.

Nigerian army spokesman Major General Chris Olukolade told the BBC the allegations of a lack of action were being made in order to discredit the military and there was no truth in them.

"This is not the first time we're hearing of hunters wanting to go into the forest. The military has always carefully utilised the support and understanding of locals ... and others who have vital knowledge and information that could enhance counter terrorism operations," he said.

Boko Haram has admitted capturing the girls, saying they should not have been in school and should get married instead.

Boko Haram, whose name means "Western education is forbidden" in the Hausa language, began its insurgency in Borno state in 2009.

At least 1,200 people are estimated to have died in the violence this year alone.